Fast handover method for IPv6 over 802.16 network

ABSTRACT

A handover method of a mobile station (MS) in a mobile communication system having MSs and radio access stations (RASs), each of which includes an IEEE 802.16 standard-based medium access control (MAC) layer and an Internet protocol version 6 (IPv6)-based IP layer. The handover method includes the step of gathering IP network information of a neighbor RAS through a message exchange with a previous RAS. A target RAS for handover based on the gathered IP network information of the neighbor RAS is determined, after the target RAS is determined, the previous RAS tunnels data for targeting the MS to the target RAS, Then the tunneled data are received from the target RAS.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit under 35 U.S.C. §119(e) of anapplication entitled “Fast Handover Method for IPv6 over IEEE 802.16Networks” filed in the United States Patent and Trademark Office on Jul.5, 2005 and assigned Ser. No. 60/695,875, and under 35 U.S.C. §119(a) ofapplications entitled “Fast Handover Method for IPv6 over IEEE 802.16Networks” filed in the Korean Intellectual Property Office on Jan. 27,2006, and Jul. 4, 2006 and assigned Serial Nos. 2006-9040 and2006-62486, respectively, the entire contents of both of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to IPv6 over IEEE 802.16networks. In particular, the present invention relates to a fasthandover method for IPv6 over IEEE 802.16 networks.

2. Description of the Related Art

The recent generalization of the use of mobile stations such as anotebook computer and a Personal Digital Assistant (PDA) increases theneed for high-speed wireless Internet service, and with the integrationof wireless networks and the Internet, users expect the networkenvironment in which they can freely use the Internet anytime anyplace.Accordingly, a Wireless Broadband Internet (WiBro) standard, which is awireless Internet standard in which high-speed Internet access ispossible not only in the stationary state but also in the moving state,has recently been established by Korean Telecommunications TechnologyAssociation (TTA). The WiBro standard supports mobility of 60 Km/h orbelow, overcoming the limitation of the existing wired system, and is sodesigned as to seamlessly provide high-speed wireless Internet serviceindoors and/or outdoors.

A mobility support method currently available in the WiBro standard isdescribed as a Medium Access Control (MAC) protocol performed between amobile station (MS) and a Radio Access Station (RAS) when the MS movesbetween RASs. However, if an MS moves to a new Access Control Router(ACR) of another subnet, it should accept an IP Mobility SupportProtocol to maintain the current session in communication. MobileInternet Protocol version 6 (MIPv6) technology established by MIP6Working Group of Internet Engineering Task Force (IETF) is a typicalinternational standard protocol for the IP Mobility Support. Inparticular, if the improved future WiBro service environment acceptsIPv6 which is the next generation Internet protocol, the MIPv6 will gainin importance as Mobility Support Protocol.

The MIPv6 technology supports mobility by binding a Home Address (HoA)of an MS with a new Care-or-Address (CoA) generated by a network towhich the MS moved, for a Home Agent (HA), using the dual addressingsystem. In particular, the MIPv6 technology can support an optimizedrouting path for data packets by sending the binding message even to aCorrespondent Node (CN). However, MIPv6, which is a protocol simplyrelated to location registration of an MS and route reestablishment fordata packets of the current session in communication, has severalproblems in supporting the mobility enough to satisfy real-timecommunication, like Voice over IP (VoIP).

MIPv6 Signaling and Handoff Optimization (MIPSHOP) Working Group of IETFhas established a Fast Mobile IPv6 (FMIPv6) protocol to make up for thedefects of MIPv6 and support fast IPv6 handover. FMIPv6 is a protocoldesigned such that it supports a MAC Layer and detects the position towhich an MS will newly move and previously exchanges informationnecessary for IPv6 handover and service resumption, thereby enablingfast service resumption when the movement actually occurs. FMIPv6 needsa definite mechanism for supporting events from the MAC layer andexchanging such events because it predicts mobility of an MS basicallydepending on the MAC layer. In addition, because the informationavailable in the link layer differs according to link type and thetiming at which the information is provided is also dependent on aprotocol of the link layer, when FMIPv6 is applied to the actualnetwork, the FMIPv6 should be redesigned such that interaction with thelink layer is optimized, taking a characteristic of the correspondinglink into account.

SUMMARY OF THE INVENTION

An aspect of exemplary embodiments of the present invention is toaddress at least the above problems and/or disadvantages and to provideat least the advantages described below. Accordingly, an object of anexemplary embodiment of the present invention is to provide a handovermethod for minimizing, by an IPv6 MS, a handover delay time duringmovement (or handover) between subnets in a WiBro network.

It is another object of an exemplary embodiment of the present inventionto provide a fast handover method for providing seamless handover evenfor real-time traffics of an MS by defining an interworking mechanismbetween an FMIPv6 IP layer and a WiBro MAC layer, and available messagestherefore.

According to an aspect of exemplary embodiments of the presentinvention, a handover method of a mobile station (MS) in a mobilecommunication system composed of MSs and radio access stations (RASs) isprovided. Each of them includes an IEEE 802.16 standard-based mediumaccess control (MAC) layer and an Internet protocol version 6(IPv6)-based IP layer. The handover method comprises the steps ofgathering IP network information of a neighbor RAS through a messageexchange with a previous RAS, determining a target RAS for handoverbased on the gathered IP network information of the neighbor RAS, afterthe target RAS is determined, tunneling, by the previous RAS, datatargeting the MS to the target RAS, and receiving the tunneled data fromthe target RAS.

Other objects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1A a message flow diagram illustrating a predictive mode handoverprocess in a conventional FMIPv6 system;

FIG. 1B is a message flow diagram illustrating a reactive mode handoverprocess in a conventional FMIPv6 system;

FIG. 2 is a message flow diagram illustrating a link layer handoverprocedure in a conventional WiBro network;

FIGS. 3A and 3B are diagrams illustrating possible configurations of aWiBro network equipped with FMIPv6 according to an exemplary embodimentof the present invention;

FIG. 4 is a message flow diagram illustrating a scenario in which an MSperforms handover in the predictive mode according to an exemplaryembodiment of the present invention;

FIG. 5 is a message flow diagram illustrating a scenario in which an MSperforms handover in the reactive mode according to an exemplaryembodiment of the present invention;

FIG. 6A is a diagram illustrating a configuration of a Layer-3 subnetaccording to an exemplary embodiment of the present invention;

FIG. 6B is a diagram illustrating movement pattern and probability of anMS according to an exemplary embodiment of the present invention;

FIG. 7 is a state transition diagram for a movement shape of an MS basedon a random walk model according to an exemplary embodiment of thepresent invention;

FIG. 8 is a diagram illustrating RAS areas, subnet areas, and a movementpattern of an MS according to an exemplary embodiment of the presentinvention;

FIG. 9 is a diagram illustrating performance evaluation on FMIPv6 overIEEE 802.16 operating in the predictive mode according to an exemplaryembodiment of the present invention;

FIG. 10 is a diagram illustrating performance evaluation on FMIPv6 overIEEE 802.16 operating in the reactive mode according to an exemplaryembodiment of the present invention;

FIG. 11 is a graph illustrating average handover delay times of thepredictive mode and the reactive mode of FMIPv6 using parameter valuesaccording to an exemplary embodiment of the present invention;

FIG. 12 is a graph illustrating the analysis result on how the MS/ACRinner processing delay time among the parameters according to anexemplary embodiment of the present invention affects the total handoverdelay time;

FIG. 13 is a graph illustrating the analysis result on the change in theH_(P)/H_(R) value for a parameter D₄, i.e. a packet forwarding time fromthe MS to the NACR (or from the NACR to the MS);

FIG. 14 is a graph illustrating the analysis result on the change in theH_(P)/H_(R) value for the parameter D₆, i.e. the uniqueness test delaytime for the NCoA, performed in the NACR; and

FIG. 15 is a graph illustrating the analysis result on the change in theH_(P)/H_(R) value for the parameter D₆, i.e. the packet arrival timefrom the NACR to the PACR (or from the PACR to the NACR) according to anexemplary embodiment of the present invention.

Throughout the drawings, the same reference numerals will be understoodto refer to the same elements, features, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The matters defined in the description such as a detailed constructionand elements are provided to assist in a comprehensive understanding ofthe embodiments of the invention. Accordingly, those of ordinary skillin the art will recognize that various changes and modifications of theembodiments described herein can be made without departing from thescope and spirit of the invention. In the following description, adetailed description of known functions and configurations incorporatedherein has been omitted for clarity and conciseness.

An operation mode of FMIPv6, which is based on mobility prediction asdescribed above, is divided into “Predictive Mode” and “Reactive Mode”depending on success/failure in the mobility prediction and handoverpreparation process.

Predictive Mode

FIG. 1A a message flow diagram illustrating a predictive mode handoverprocess in a conventional FMIPv6 system.

Referring to FIG. 1A, upon detecting a New Radio Access Station (NRAS),a mobile station (MS) 10 sends a Router Solicitation for Proxy (RtSolPr)message to a Previous ACR (PACR) 20 from which it is currently receivinga service in order to acquire information on a New ACR (NACR) 30 towhich the NRAS (not shown) belongs in step S101. Upon receipt of theRtSolPr message, the PACR 20 detects IP address, MAC address and subnetprefix information of the NACR 30 based on a MAC address of a newneighbor RAS, extracted from the RtSolPr message, and sends a ProxyRouter Advertisement (PrRtAdv) message including the detectedinformation to the MS 10 in step S102.

In this manner, the MS 10 gathers information on neighbor RASs (notshown) and the NACR 30, and determines a target RAS (i.e. NRAS) to whichit will perform handover based on the gathered information. Henceforth,a neighbor ACR to which the target RAS belongs becomes a target ACR(i.e. NACR). The MS 10 creates a New Care-of-Address (NCoA), or a new IPaddress, to be used after handover based on prefix information of theNACR 30, acquired through the PrRtAdv message, and sends a Fast BindingUpdate (FBU) message to the PACR 20 to bind a Previous Care-of-Address(PCoA) with the NCoA in step S103.

Upon successful receipt of a Fast Binding Acknowledgement (FBAck)message in response to the FBU message in step S106, the MS 10 operatesin the predictive mode after handover.

Upon receipt of the FBU message from the MS 10, the PACR 20 sends aHandover Initiation (HI) message to the NACR 30 in step S104, andreceives a Handover Acknowledge (HAck) message in response thereto instep S105. Then the PACR 20 verifies uniqueness of an NCoA to be usedafter the movement (or handover) using the HAck message, and sends anFBAck message with the NCoA to the MS 10 and the NACR 30 in step S106.In addition, the PACR 20 creates a PCoA-NCoA tunnel and tunnels allpackets targeting the PCoA to the NCoA in step S107. Then the NACR 30intercepts the packets targeting the NCoA and buffers the interceptedpackets therein. Thereafter, upon receipt of a Fast NeighborAdvertisement (FNA) message from the MS 10 after completion its movementin step S108, the NACR 30 forwards all the buffered packets to the MS 10in step S109, completing the handover procedure.

Reactive Mode

FIG. 1B is a message flow diagram illustrating a reactive mode handoverprocess in a conventional FMIPv6 system.

Referring to FIG. 1B, after steps S111 and S112 similar to thecorresponding steps in FIG. 1A, an MS 10 operates in a reactive mode instep S113, if it fails to send an FBU message before handover, or if itstarts handover before receipt of an FBAck message even though it sentthe FBU message.

In this case, the MS 10, as it fails to receive the FBAck message,cannot determine whether the FBU message has normally arrived at a PACR20. Therefore, the MS 10 encapsulates the FBU message in an FNA messageand sends the FNA message to an NACR 30 in step S114. Upon receipt ofthe FBU message from the MS 10, the NACR 30 first determines whether anNCoA included therein is identical to the address used in thecorresponding network. If the addresses are not identical to each otheras a result of the uniqueness check, the NACR 30 sends the FBU messageincluded in the FNA message to the PACR 20 in step S115. Upon arrival ofan FBAck message, the NACR 30 generates a PCoA-NCoA tunnel and finallyforwards the tunneled packets to the MS 10 in step S117. In this case,the FBAck message, because its destination is the NCoA, is forwarded tothe MS 10 together with the tunneled packets. If the NCoA is already inuse, the NACR 30 sends a PrRtAdv message including a negativeacknowledgement (NACK) message to the MS 10 and discards the FBU messagein step S119.

FIG. 2 is a message flow diagram illustrating a link layer handoverprocedure in a conventional WiBro network.

Referring to FIG. 2, the handover in the WiBro network conceptuallycomprises a neighbor network search/information acquisition phase, ahandover preparation phase, and a handover execution phase.

(1) Neighbor Network Search/Information Acquisition Phase

Compared with the network entry/handover procedure in the wireless LocalArea Network (LAN), the handover procedure in the WiBro network iscomposed of more complex processes to support accurate adjustment ofparameters and flexibility in the procedure. A WiBro MS 10 receives aNeighbor Advertisement (MOB_NBR-ADV) message that is periodicallyadvertised from its RAS in a corresponding network in step S201. TheMOB_NBR-ADV message comprises network attributes for a PRAS 21 andneighbor RASs. Therefore, upon receipt of this message, the MS 10 canacquire identifier (ID), quality-of-service (QoS) parameter, and channelinformation of the neighbor RASs, and later use the acquired informationto perform faster handover.

There is another network information acquisition method that uses ascanning procedure performed by the MS 10. This method refers to theprocedures S202, S203 and S204 for measuring signal qualities ofdownlinks received from the neighbor RASs. The MS 10 can acquire an IDlist of the neighbor RASs through the MOB_NBR-ADV message, select anappropriate RAS based on real-time link information acquired through thescanning, and manage a list of candidate RASs for handover.

To reduce a handover delay time, the MS 10 can perform an associationprocess including ranging with the neighbor RASs in the scanningprocess. The ranging refers to the procedure that the MS 10 firstperforms in the course of entering a new network. Through this process,the MS 10 acquires the basic information related to physicalcharacteristic, timing and power control of the channel, including thefrequency used in an NRAS 31, thereby accelerating the handoverprocedure.

(2) Handover Preparation Phase

The MS 10 determines an optimal NRAS 31 by comparing the alreadyacquired signal strengths and QoS parameters of neighbor RASs. The MS 10compares QoS and signal strength provided from a PRAS 21 with associatedthresholds to create a list of RASs for handover, and sends an MSHandover Request (MOB_MSHO-REQ) message with the RAS list to the PRAS 21in step S205. In response thereto, the PRAS 21 comprises a candidate RASlist acquired depending on the RAS list in a BS Handover Response(MOB_BSHO-RSP) message, and sends the MOB_BSHO-RSP message to the MS 10in step S206. In this case, the PRAS 21 can previously send session andconfiguration information of the MS 10 by notifying the handover to thecandidate RASs via a backbone network, thereby reducing the futurehandover time. In this phase, the PRAS 21 can first send a BS HandoverRequest (MOB_BSHO-REQ) message to the MS 10 in step S207, therebyinitiating the handover process.

(3) Handover Execution Phase

If the MS 10 determines a target RAS and is ready to move thereto, theMS 10 can no longer exchange packets via the PRAS 21 from the time atwhich it sends a Handover Indication (MOB_HO-IND) message to the PRAS 21in step S208. After its movement, the MS 10 performs a network entryprocess in step S209. The MS 10 first performs ranging to acquire linksynchronization with the NRAS 31. After successfully completing theranging, the MS 10 enters capability negotiation with the NRAS 31.Thereafter, the MS 10 finally registers itself in the NRAS 31 through anauthentication process. If the NRAS 31 has previously received thecapability and authentication information of the MS 10 via the backbonenetwork, the MS 10 can omit the corresponding process, reducing thehandover process. If the network entry process is successfully completedafter completion of the registration process, the NRAS 31 can start aservice to the MS 10 from that time on in step S210.

If the MS 10 moves to another subnet in the network, it should reacquirea valid NCoA and additionally follow an IP access reestablishmentprocess using the valid NCoA. In addition, to resume the sessionperformed in the previous network using the NCoA, the MS 10 shouldseparately perform an IP handover procedure, like MIPv6.

FIGS. 3A and 3B are diagrams illustrating possible configurations of aWiBro network equipped with FMIPv6 according to an exemplary embodimentof the present invention.

Referring to FIG. 3A, the network is divided into two subnetsrepresented by ACR, and each ACR manages a plurality of RASs. In thiscase, not every inter-RAS movement always requires IPv6 mobilitymanagement. Because inter-RAS movement in a particular ACR becomesmovement in the same subnet, it is also possible to maintain thecommunication using only the handover based on the WiBro standardwithout the IPv6 mobility management. However, in the case where an MSmoves to a new subnet, like in the case where the MS moves from a RAS5to a RAS6 in FIG. 3A, IPv6 mobility management and handover should besupported to maintain the session.

Referring to FIG. 3B, in the network, ACRs and RASs are mapped to eachother on a one-to-one basis. In this case, the RAS and the ACR can bephysically integrated into one network equipment. In this environment,because random movement of the MS always means movement to a new ACR, amobility support protocol like MIPv6 should be supported to maintain theold session. Accordingly, the FMIPv6-based handover scheme proposed inthe present invention can be applied to the case where the MS moves to anew subnet, i.e. the case where the MS moves to a new ACR in FIG. 3A,and to every movement in FIG. 3B.

Herein, a description of a tight interworking operation between a linklayer and an IP layer and an optimized handover procedure based onFMIPv6 for minimizing a handover delay time according to the presentinvention will be separately made for the predictive mode and thereactive mode.

NRAS Information Acquisition

Once a target RAS candidate is determined using the neighbor networksearch/information acquisition method described in FIG. 2, the MS shouldacquire information on an ACR associated with the target RAS. The MS canacquire the information on the ACR connected to the target RAS throughan exchange of the RtSolPr and PrRtAdv messages, which are Layer-3messages proposed in FMIPv6. In order to obtain the information on theassociated ACR as quickly as possible by immediately notifying detectionof a new RAS in Layer 2 (L2) to Layer 3 (L3), the present inventiondefines New_RAS_Found as an inner primitive in the MS, as follows.

New_RAS_Found (New RAS IDs)

Sender: Layer 2 in MS

Recipient: Layer 3 in MS

Action during transmission: new target RAS candidate is found

Action during reception: RtSolPr message is sent to PACR

Upon receipt of a New_RAS_Found message including IDs of new RASs, Layer3 in the MS comprises the IDs in an RtSolPr message and rapidly sendsthe RtSolPr message to the PACR. Upon receipt of the RtSolPr message,the PACR sends a PrRtAdv message including a list of [BSID, ACR-Info] inresponse thereto. The ACR-Info comprises an IP and a MAC address of theNACR, and a prefix managed by the NACR. A process of gatheringinformation on neighbor RASs, selecting a candidate RAS, and acquiringinformation on the NACR associated with the candidate RAS should not benecessarily performed at particular timing in the WiBro handoverprocedure, and the MS can perform the process at its convenient timelike the idle time.

Handover Proposal

Next, a detailed description will be made of handover optimized throughtight interworking between a WiBro standard for Layer 3 and a WiBrostandard for Layer 2. The proposed handover procedure is roughly dividedinto (1) movement (or handover) preparation phase, (2) movement (orhandover) execution phase, (3) WiBro network entry phase, and (4)movement (or handover) completion phase.

(1) Movement Preparation Phase

The MS finally determines handover to a target RAS and sends aMOB_MSHO-REQ message thereto. Upon receipt of an MS Handover Response(MOB_MSHO-RSP) message from the RAS, the MS starts the WiBro handoverprocedure in Layer 2. In order to minimize the handover time, Layer 2 ofthe MS should send a FBU message to the PACR without delay, afterreceiving MOB_BSHO-RSP/MOB_BSHO-REQ messages.

Conventionally, however, because the WiBro handover process of Layer 2and the fast handover mechanism for Layer 3 independently operate, ifthe FBU message is sent before the MOB_BSHO-RSP/MOB_BSHO-REQ messagesare received, or if FMIPv6, which is Layer 3, fails to detect receipt ofthe MOB_BSHO-RSP or MOB_BSHO-REQ message though it is received, sendingof the FBU message is delayed causing a delay in the overall handoverprocedure. Therefore, there is a need for the following new primitiveused for notifying receipt of the MOB_BSHO-RSP/MOB_BSHO-REQ messages inLayer 2 to Layer 3.

Link_Going_Down

Sender: Layer 2 in MS

Recipient: Layer 3 in MS

Action during transmission: MOB_BSHO-RSP or MOB_BSHO-REQ message isreceived

Action during reception: FBU message is sent to PACR

Upon receipt of the FBU message from the MS, the PACR generates aPCoA-NCoA tunnel through an exchange of HI and HAck messages with theNACR, and tunnels data packets to the NCoA immediately after sending anFBAck message to the MS. The HAck message comprises uniquenessinformation for the NCoA, which is an address to be used in the targetnetwork.

According to the WiBro standard, if the MS sends a MOB_HO-IND message tothe PRAS and the PRAS receives this message, the communication betweenthe MS and the RAS is no longer permitted, even though a resourceholding timer for the corresponding MS does not expire in the RAS.Therefore, in order to operate in the predictive mode, the MS shouldsuccessfully exchange the FBU and FBAck messages with the PACR beforesending the MOB_HO-IND message.

(2) Movement Execution Phase

If the MS has successfully received an FBAck message before handoverexecution and received uniqueness for the NCoA through the message, theMS should rapidly perform Layer-2 handover. In particular, because thePACR is already tunneling packets to the NCoA, the MS should performhandover as quickly as possible. This Layer-2 handover is initiated asthe MS sends the MOB_HO-IND message to the PRAS. In order to allow theMS to send the MOB_HO-IND message immediately after receiving the FBAckmessage and to perform handover as quickly as possible, the presentinvention defines the following primitive.

Link_Switch

Sender: Layer 3 in MS

Recipient: Layer 2 in MS

Action during transmission: FBAck message is received

Action during reception: MOB_HO-IND message is sent to PRAS, andmovement is performed

This primitive, a kind of the command sent from Layer 3 to Layer 2, hasa function of allowing Layer 3 of the MS to send a MOB_HO-IND messageafter receiving an FBAck message. That is, in order to help FMIPv6operate in the predictive mode if possible, Layer 2 of the MS shoulddelay sending of the MOB_HO-IND message until it receives the definedLink_Switch command.

Even though the MS fails to receive the FBAck message before sending theMOB_HO-IND message, if it moves at very high speed or quality of adownlink signal from the corresponding RAS abruptly deteriorates, the MScannot but rapidly send the MOB_HO-IND message. In this case, FMIPv6operates in the reactive mode. That is, the Link_Switch is used by theMS to rapidly start its movement immediately after movement preparationis completed in Layer 3, or to hold start of movement in Layer 2 untilmovement preparation of Layer 3 is completed. The use of the Link_Switchreduces the handover delay time and increases the probability ofoperating in the predictive mode if possible.

However, if a drop of the service is expected due to the abruptdeterioration of the signal quality from the corresponding RAS, the MSshould start its movement to the NRAS even though Layer 3 of the MS hasnot completed the movement preparation. In this case, the Link_Switchmessage is not used, and the MS operates in the reactive mode after thehandover.

(3) New Network Entry Phase

If the MS moves to a new network, it acquires synchronization with theNRAS and performs a network entry procedure. In this phase, the MSexchanges the RNG-REQ/RSP, SBC-REQ/RSP, PKM-REQ/RSP, REG-REQ/RSPmessages with the NRAS. If the NRAS has already received session andconfiguration information of the MS from the PRAS before or duringhandover, a message exchange for the corresponding information can beomitted. With the completion of the network entry procedure, thehandover process in Layer 2 is completed.

Immediately after completing the handover process, Layer 2 sends thefollowing primitive to Layer 3.

Link_Up

Sender: Layer 2 in MS

Recipient: Layer 3 in MS

Action during transmission: network entry procedure is completed

Action during reception: FNA message is sent to NACR

Using the Link_Up, Layer 2 of the MS notifies to Layer 3 the possibilityof exchanging packets through a link as Layer-2 handover is fullycompleted, and upon receipt of the notification, Layer 3 sends an FNAmessage to the NACR. In the reactive mode, Layer 3 should comprise theFBU message in the FNA message should.

(4) Handover Completion Phase

Upon receipt of the FNA message from the MS, the NACR sends thetunneling packets buffered therein during handover to the MS in thepredictive mode. In the reactive mode, the NACR extracts the FBU messageincluded in the received FNA message, sends the extracted FBU message tothe PACR to establish a PCoA-NCoA tunnel, and finally forwards thepackets received via the tunnel to the MS. In the reactive mode, theNACR should perform uniqueness check on the NCoA included in the FBUmessage.

Handover Scenario

This section shows the scenario in which the MS performs handoveraccording to the proposed handover procedure in the predictive mode andthe reactive mode.

FIG. 4 is a message flow diagram illustrating a scenario in which an MSperforms handover in the predictive mode according to an exemplaryembodiment of the present invention.

A PRAS 21 periodically broadcasts a MOB_NBR-ADV message in step S401.Upon discovering new neighbor RASs in this message, Layer-2 12 of an MSsends a New_RAS_Found message to Layer-3 13 of the MS in step S10.Before this, the MS can perform scanning to obtain detailed linkinformation in step S402. Upon receipt of the New_RAS_Found message,Layer-3 13 of the MS acquires information on the ACR connected to thenewly discovered RAS by exchanging RtSolPr and PrRtAdv messages with aPACR 22 in steps S403 and S404.

If the MS determines handover, it exchanges MOB_MSHO-REQ andMOB_BSHO-RSP messages with the PRAS 21, and selects a target RAS, i.e.NRAS, in steps S405 and S406. Alternatively, the PRAS 21 can starthandover by sending a MOB_BSHO-REQ message to the MS in step S407. Uponreceipt of a MOB_BSHO-RSP or MOB_BSHO-REQ message from the PRAS 21 inresponse to the MOB_MSHO-REQ message in step S406 or S407, Layer-2 12 ofthe MS generates Link_Going_Down to notify the receipt to Layer-3 13 instep S20. Upon receipt of the Link_Going_Down, Layer-3 13 of the MSexchanges FBU and FBAck messages with the PACR 22 in steps S408 andS411. Prior to sending the FBAck message, the PACR 22 exchanges HI andHAck messages with an NACR 32 to establish a tunnel in steps S409 andS410. The NACR 32 notifies uniqueness of the NCoA through the HAckmessage. At this moment, the packets are tunneled to the NCoA.

Upon receipt of the FBAck message in step S411, the MS allows Layer 2 tosend a MOB_HO-IND message, using Link_Switch in steps S30 and S412.

The MS performs the network entry procedure according to the WiBrostandard in step S413. After completion of the network entry procedure,Layer-2 12 of the MS generates Link_Up to notify the completion toLayer-3 13 in step S40, and Layer-3 13 immediately sends an FNA messageto the NACR 32 in step S414. Upon receipt of the FNA message from theMS, the NACR 32 starts forwarding of the tunneling packets buffered forthe corresponding MS in step S415.

FIG. 5 is a message flow diagram illustrating a scenario in which an MSperforms handover in the reactive mode according to an exemplaryembodiment of the present invention.

A PRAS 21 periodically broadcasts a MOB_NBR-ADV message in step S501.Upon discovering new neighbor RASs in this message, Layer-2 12 of an MSsends a New_RAS_Found message to Layer-3 13 of the MS in step S10.Before this, the MS can perform scanning to obtain detailed linkinformation in step S502. Upon receipt of the New_RAS_Found message,Layer-3 13 of the MS acquires information on the ACR connected to thenewly discovered RAS by exchanging RtSolPr and PrRtAdv messages with aPACR 22 in steps S503 and S504.

If the MS determines handover, it exchanges MOB_MSHO-REQ andMOB_BSHO-RSP messages with the PRAS 21, and selects a target RAS, i.e.NRAS, in steps S505 and S506. Alternatively, the PRAS 21 can starthandover by sending a MOB_BSHO-REQ message to the MS in step S507. Uponreceipt of a MOB_BSHO-RSP or MOB_BSHO-REQ message from the PRAS 21 inresponse to the MOB_MSHO-REQ message in step S506 or S507, Layer-2 12 ofthe MS generates Link_Going_Down to notify the receipt to Layer-3 13 instep S20. Upon receipt of the Link_Going_Down, Layer-3 13 of the MSimmediately sends a FBU message to the PACR 22. In step S509, the MSoperates in the reactive mode, if it cannot send the FBU message, or ifit fails to receive an FBAck message in response to the FBU message eventhough it has sent the FBU message.

The MS performs the network entry procedure according to the WiBrostandard in step S510. After completion of the network entry procedure,Layer-2 12 of the MS generates Link_Up to notify the completion toLayer-3 13 and Layer-3 13 immediately sends an FNA message to the NACR32 in step S511. The MS comprises the FBU message in the FNA messagebecause it operates in the reactive mode.

Upon receipt of the FNA message, the NACR 32 checks uniqueness of theNCoA included therein. If the NCoA is unique, the NACR 32 generates atunnel by exchanging FBU and FBAck messages with the PACR 22, andfinally sends the tunneling packets to the MS in steps S512, S513 andS514. If the NCoA is already in use, the NACR 32 sends a PrRtAdv messageincluding a NACK message to the MS and discards the FBU message in stepS515.

For performance analysis of the present invention, a definition will begiven of a packet-level traffic model, a system model, and MS mobilitymodel. With the use of these models, performance of FMIPv6 will beanalyzed hereinbelow. The important analysis criterion is an L2/L3handover delay time required for one movement.

System and Mobility Model

FIG. 6A is a diagram illustrating a configuration of a Layer-3 subnetaccording to an exemplary embodiment of the present invention, and FIG.6B is a diagram illustrating movement pattern and probability of an MSaccording to an exemplary embodiment of the present invention.

Referring to FIGS. 6A and 6B, one subnet is generally composed of morethan one RAS areas. For convenience, it is assumed that every RAS areahas a mesh-type rectangular structure having the same shape and size. Itis also assumed that an MS moves along a 2-dimensional Random WalkModel. The present invention compares signaling cost and handover timefor only one MS without taking into account the MSs scattered under theRAS and the ACR.

If a particular subnet is composed of N=4n²−4n+1 RAS areas, this subnetis called an n−layer subnet. FIG. 6A shows a 3-layer subnetconfiguration. Subnets are named layer 0, layer 1, layer 2 . . . ,drawing a ring from the center, and a RAS area surrounding a layer x−18RAS area is called layer x RAS area. An n-layer subnet is composed of alayer 0 RAS area to a layer n−1 RAS area.

Assuming that after staying in one RAS area for a certain time, an MSmoves to one of 4 neighbor RAS areas at the same probability (i.e. ¼) asshown in FIG. 6B, all RAS areas in one subnet can be divided intoseveral RAS area <x,y> groups. The RAS area groups are distinguished byshape. Herein, x indicates that the RAS area is in layer x, and yindicates that a corresponding group is a (y+1)^(th) group in layer x.The RAS areas having the same shape show the same movement pattern. FIG.6A shows the shapes of the RAS areas for the 3-layer subnet.

FIG. 7 is a state transition diagram for a movement shape of an MS basedon a random walk model according to an exemplary embodiment of thepresent invention.

Referring to FIG. 7, in the random walk model, a state (x,y) indicatesthat a particular MS is located in one of RAS areas of an <x,y> group.For 0≦j≦2n−3, an absorbing state (n,j) means that the MS leaves thecorresponding subnet from the state. A transition matrixP=(p_((x,y)(x′,y′))) for the random walk state transition diagram isexpressed as

$\begin{matrix}{P = \begin{pmatrix}0 & 0 & 1 & 0 & 0 & \cdots & 0 & 0 & 0 \\0 & 0 & \frac{1}{2} & 0 & \frac{1}{4} & \cdots & 0 & 0 & 0 \\\frac{1}{4} & \frac{1}{2} & 0 & 0 & 0 & \cdots & 0 & 0 & 0 \\0 & 0 & 0 & 0 & \frac{1}{4} & \cdots & 0 & 0 & 0 \\\vdots & \vdots & \vdots & \vdots & \vdots & ⋰ & \vdots & \vdots & \vdots \\0 & 0 & 0 & 0 & 0 & \cdots & 1 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & \cdots & 0 & 1 & 0 \\0 & 0 & 0 & 0 & 0 & \cdots & 0 & 0 & 1\end{pmatrix}} & (1)\end{matrix}$

With the use of the matrix P, a matrix P^((k)) can be found using anChapman-Kolmogorov equality. For k≧1, a particular element p_((x) ^(k)_(,y)(x′,y′)) of P^((k)) means the probability that the MS will movefrom a state (x,y) to a state (x′,y′) exactly in k steps.

Using the two matrixes P and P^((k)), p_(k,(x,y),(n,j)) can be finallydefined as

$\begin{matrix}{p_{k,{{({x,y})}{({n,i})}}} = \left\{ \begin{matrix}p_{{({x,y})}{({n,i})}} & {{{for}\mspace{14mu} k} = 1} \\{p_{{({x,y})}{({n,i})}} - p_{{({x,y})}{({n,i})}}^{k - 1}} & {{{for}\mspace{14mu} k} > 1}\end{matrix} \right.} & (2)\end{matrix}$

From Equation (2), p_(k,(x,y),(n,j)) means the probability that an MS,which was initially located in one of RASs of an <x,y> group, will moveto a RAS of an <n−1,j> group in a k−1^(th) step, and then leave thecorresponding subnet in the last step.

For 0≦j≦2n−3, q_((n−1,j)) is defined as the probability that the MS willenter the subnet via a RAS area with a <n−1, j> shape. Similarly, {tildeover (q)}_((n−1,j)) is defined as the probability that the MS will leavethe subnet via a RAS area with a <n−1,j> shape. For n>2 and 0≦j≦2n−3,

$\begin{matrix}{{{\sum\limits_{i = 0}^{{2n} - 3}{\overset{\sim}{q}}_{({{n - 1},i})}} = 1}\mspace{20mu}{{\overset{\sim}{q}}_{({{n - 1},i})} = {\sum\limits_{k = 1}^{\infty}{\sum\limits_{y = 0}^{{2n} - 3}{q_{({{n - 1},y})}p_{k,{{({{n - 1},y})}{({n,i})}}}}}}}} & (3)\end{matrix}$

In Equation (3), the right detailed formulas mean the product of theprobabilities that the MS, which was initially located in an <n−1,y> RASarea, leaves its current subnet in the <n−1,y> RAS area after k^(th)movement. From FIG. 6A and FIG. 8 that shows RAS areas, subnet areas,and a movement pattern of an MS according to an exemplary embodiment ofthe present invention, it can be noted thatq _(n−1,j)) =q _((n−1,j.)  (4)

From the foregoing equations, a linear system and a solution forq_(n−1,j)) can be obtained. For a 2-layer subnet, q(1,0)≅66.667% andq(1,1)≅33.333% can be obtained. For a 3-layer subnet, q(2,0)≅40%, andfor 0<j≦3, q (2,j)≅20% can be obtained. In addition, for a 4-layersubnet, q (3,0)≅28.571% and for 0<j≦5, q(3,j)≅14.285%. Finally, for a5-layer subnet, q(4,0)≅22.222%, and for 0<j≦7, q(4,j)≅11.111% can befound. These probabilities can be extended up to a particular layer inthe same method. In finding the solution, 500 terms are summed up forthe sum up to the infinite. In this case, an error is below 10⁻¹⁷.

Finally, if it is assumed that the MS moves over M RAS areas until itleaves a particular subnet after it entered the subnet, then E[M]=1 forn=1, and for n≧2, E[M] is

$\begin{matrix}{{B\lbrack M\rbrack} = {\sum\limits_{k = 1}^{\infty}{\sum\limits_{y = 0}^{{2n} - 3}{\sum\limits_{i = 0}^{{2n} - 3}{q_{({{n - 1},y})}p_{k,{{({{n - 1},y})}{({n,i})}}}k}}}}} & (5)\end{matrix}$

Performance Evaluation Result

An FMIPv6 handover delay time, which is the object of evaluation in thepresent invention, is defined as a difference between a time of the lastpacket received in the previous RAS area and a time of a first(tunneling) packet received in a new RAS area. Meanwhile, a Layer-2handover delay time disclosed in IEEE 802.16e is defined as a differencebetween the time at which the MS sends a MOB_HO-IND message in theprevious RAS area and the time at which the MS sends a Link_Up triggerto Layer 3 after completing the network entry process in the new RASarea.

To accurately find the FMIPv6 handover delay time together with thesedefinitions, a definition of the following parameters is given.

-   -   D₁: time required for sending a MOB_HO-IND message after        receiving an FBAk message and the last packet from PACR    -   D₂: Layer ½ handover delay time    -   D₃: time required for sending an FNA message after receiving a        Link_Up trigger    -   D₄: time required for forwarding packets from an MS to an NACR        (or from the NACR to the MS)    -   D₅: delay time for a uniqueness test on an NCoA, performed in        the NACR    -   D₆: time required by packets to arrive from the NACR at the PACR        (or from the PACR at the NACR)    -   D₇: time required until the MS starts data packet tunneling        after receiving the FBU message from the PACR

FIG. 9 is a diagram illustrating performance evaluation on FMIPv6 overIEEE 802.16 operating in the predictive mode according to an exemplaryembodiment of the present invention. For simple and clear expression ofthe present invention, if every handover-related signaling and data isequal in its source and destination, it is assumed that the signalingand data has the same delay time regardless of its size.

Referring to FIG. 9, when the predictive mode of FMIPv6 is used, ahandover delay time during inter-ACR movement isHandover delay time=D ₁ +D ₂ +D ₃+2D ₄  (6)

When the predictive mode of FMIPv6 is used taking into account only theinter-RAS movement as well without the inter-ACR movement, an averagehandover delay time H_(P) required during certain movement of the MS is

$\begin{matrix}\begin{matrix}{H_{P} = \frac{{\left( {{E\lbrack M\rbrack} - 1} \right)D_{2}} + D_{1} + D_{2} + D_{3} + {2D_{4}}}{E\lbrack M\rbrack}} \\{= {D_{2} + \frac{D_{1} + D_{3} + {2D_{4}}}{E\lbrack M\rbrack}}}\end{matrix} & (7)\end{matrix}$

FIG. 10 is a diagram illustrating performance evaluation on FMIPv6 overIEEE 802.16 operating in the reactive mode according to an exemplaryembodiment of the present invention.

Referring to FIG. 10, when the reactive mode of FMIPv6 is used, ahandover delay time during inter-ACR movement isD ₂ +D ₃+2D ₄ +D ₅+2D ₆ +D ₇  (8)

When the reactive mode of FMIPv6 is used taking into account only theinter-RAS movement as well without the inter-ACR movement, an averagehandover delay time H^(R) required during certain movement of the MS is

$\begin{matrix}\begin{matrix}{H_{R} = \frac{{\left( {{E\lbrack M\rbrack} - 1} \right)D_{2}} + D_{2} + D_{3} + {2D_{4}} + D_{6} + {2D_{6}} + D_{7}}{E\lbrack M\rbrack}} \\{= {D_{2} + \frac{D_{3} + {2D_{4}} + D_{6} + {2D_{6}} + D_{7}}{E\lbrack M\rbrack}}}\end{matrix} & (9)\end{matrix}$

A basic value list for each parameter to be used for performanceanalysis is shown in Table 1. These values are hypothesized values, andcan undergo various changes depending on wireless environment, networkconfiguration, and protocol realization method in MS, RAS and ACR.

TABLE 1 Parameter Value D₁ = D₃ = D₇28888888 5 ms D₂ 30 ms D₄ 2 ms D₅ 10ms D₆ 3 ms

FIG. 11 is a graph illustrating average handover delay times of thepredictive mode and the reactive mode of FMIPv6 using parameter valuesaccording to an exemplary embodiment of the present invention.

Referring to FIG. 11, it can be noted that as a size of a subnet islarger, i.e. as the number of RASs in one subnet is greater, an averagehandover delay time of the MS is shorter. This result is caused by thedecrease in the number of the cases where L3 handover of a network leveloccurs. It can be understood that when every RAS is composed of onesubnet (i.e. n=1), the handover delay times of the reactive mode and thepredictive mode are higher 2 times and 1.5 times than the Layer-½average handover delay time (30 ms), respectively. Finally, it can alsobe noted that as a size of the subnet is larger, a performancedifference between the predictive mode and the reactive mode is smaller.

FIG. 12 is a graph illustrating the analysis result on how the MS/ACRinner processing delay time among the parameters according to anexemplary embodiment of the present invention affects the total handoverdelay time.

Referring to FIG. 12, in this analysis, a radio H_(P)/H_(R) determinedby dividing the predictive mode handover delay time by the reactive modehandover delay time is defined as a metric. Therefore, it can be notedthat as the ratio H_(P)/H_(R) is lower, the predictive mode is superiorto the reactive mode in terms of the performance.

As could be understood from FIG. 10, it can be noted that as the size ofthe subnet is larger, the predictive mode is not so superior to thereactive mode in performance. That is, to maximize an effect of thepredictive mode, it is necessary to reduce the subnet size if possible.

It is also noted that the MS and the ACR should complete the innerprocessing if possible in order to increase the performance of thepredictive mode. From this result, it can be appreciated that theperformance of the predictive mode is susceptible to the innerprocessing delay time.

FIG. 13 is a graph illustrating the analysis result on the change in theH_(P)/H_(R) value for a parameter D₄, i.e. a packet forwarding time fromthe MS to the NACR (or from the NACR to the MS).

Similarly to the analysis shown in FIG. 5, it can be noted that as theD₄ value is smaller, the predictive mode is superior to the reactivemode in performance. That is, it can be understood that the performanceof the predictive mode is susceptible to the packet forwarding time fromthe MS to the NACR (or from the NACR to the MS). It can also beappreciated from FIG. 5 that there is almost no performance differencebetween the predictive mode and the reactive mode in the environmentwhere Layer-½ handover is much greater than Layer-3 handover in thenumber of handover occurrences due to the considerably large size of thesubnet.

The analysis result on the change in the H_(P)/H_(R) value for theparameter D₆, i.e. the uniqueness test delay time for the NCoA,performed in the NACR, is illustrated in FIG. 14.

FIG. 14 is a graph illustrating the analysis result on the change in theH_(P)/H_(R) value for the parameter D₆, i.e. the uniqueness test delaytime for the NCoA, performed in the NACR.

Referring to FIG. 14, D₆ greatly affects the predictive mode andreactive mode performances. The reason is because the uniqueness testdelay time for the NCoA, performed in the NACR, is a factor of thehandover delay time in the reactive mode, whereas the uniqueness testdelay time is not the factor of the handover delay time in thepredictive mode. Therefore, it can be appreciated that as the D₆ valueis greater, the predictive mode is much superior to the reactive mode inperformance. It is remarkable that that unlike the parameters D₁, D₃, D₄and D₇, the D₆ value affects the H_(P)/H_(R) value regardless of thesubnet size.

FIG. 15 is a graph illustrating the analysis result on the change in theH_(P)/H_(R) value for the parameter D₆, i.e. the packet arrival timefrom the NACR to the PACR (or from the PACR to the NACR) according to anexemplary embodiment of the present invention.

It can be noted from FIG. 15 that similarly to the analysis result onthe D₆, the D₆ value also affects the H_(P)/H_(R) value regardless ofthe subnet size. The D₆ is a factor of the handover delay time of thereactive mode. However, the D₆ is not the factor of the handover delaytime in the predictive mode. Therefore, as the D₆ value is greater, thepredictive node is higher than the reactive mode in the performance,

Summarizing the present invention, the WiBro advocates handovertechnology for supporting a moving user as well as the conventionalfixed user, as its core function. Therefore, the standard specificationfor movement between two RASs under a single ACR (single subnet) isalready well defined. However, there is no standard mentioning in detailthe IP handover technology between two ACRs (different subnets), whichshould be taken into account when performing IPv6 service in the WiBronetwork.

The present invention describes the detailed technology related to thenetwork search, movement preparation, movement execution, network entryand movement completion, which should be taken into account whenapplying FMIPv6 that is IPv6 handover technology recently established asRFC in IETF, to the WiBro network. In particular, the present inventionproposes the primitives used for interworking between Layer 2 and Layer3, necessary for every handover execution step, as shown in Table 2.

In addition, the present invention proposes how to efficiently operatethe FMIPv6 handover procedure and the WiBro handover procedure usingthese primitives separately for the predictive mode and the reactivemode, and presents the detailed scenario.

Finally, in the performance evaluation section, the present inventionderived the delay time parameters while redeveloping the overallhandover process along the time axis, and analyzed the handover delaytime of FMIPv6 in the WiBro network while hypothesizing an appropriatevalue for each of the derived parameters. Although it may undergo aconsiderable change according to parameter value, it could be noted fromthe analysis result that when FMIPv6 is used under an appropriateparameter value, the integrated Layer-2/Layer-3 handover delay time isdistributed over 30˜40 ms for the predictive mode, and 35˜60 ms for thereactive mode. In addition, it was verified that compared with thereactive mode, the predictive mode could reduce the handover delay timeby almost 5˜30% according to several parameter values.

TABLE 2 Tx Primitive direction Description New_RAS_Found L2−>L3 Becausea new RAS is discovered, Layer 2 notifies to Layer 3 the necessary ofacquiring information on the ACR connected to the RAS. Link_Going_DownL2−>L3 Because it is determined that handover is imminent due to thegradual decrease in the strength of a signal detected in the link, Layer2 notifies to Layer 3 the need for preparing the handover procedure.Link_Switch L3−>L2 Because Layer-3 handover preparation is completed,Layer 3 instructs Layer 2 to perform substantial handover. Link_UpL2−>L3 After the handover, Layer 2 notifies to Layer 2 the possibilityof using the link after completing the network entry process in the newlink.

As can be understood from the foregoing description, the handover methodaccording to the present invention defines the primitives forinterworking between the MAC layer and the IP layer, and optimizes theFMIPv6 handover procedure and the WiBro handover procedure using theseprimitives, thereby minimizing the handover delay time of the MS.

While the invention has been shown and described with reference to acertain exemplary embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A handover method of a mobile station (MS) in a mobile communicationsystem including MSs, radio access stations (RASs), and access controlrouters (ACRs), each of which includes an IEEE 802.16 standard-basedmedium access control (MAC) layer and an Internet protocol version 6(IPv6)-based IP layer, the method comprises: gathering IP networkinformation of a neighbor RAS through a message exchange with a previousACR; determining a target RAS for handover based on the gathered IPnetwork information of the neighbor RAS gathered from the previous ACR;after the target RAS is determined, tunneling, by the previous ACR, datafor targeting the MS to the target RAS; and receiving the tunneled datafrom the target RAS, wherein the target RAS determining comprisesselecting the target RAS through MAC message exchange between the MS andthe previous RAS, if the target RAS is selected, generating a target RASselect primitive and forwarding the target RAS select primitive to an IPlayer, sending, by the MAC layer of the MS, a Handover InitiationMOB_HO-IND message to the previous RAS; upon receipt of the MOB_HO-INDmessage, ending, by the previous RAS, service to the MS, upon receipt ofthe target RAS select primitive, sending, by the IP layer, a fastbinding update FBU message to the upper node of an ACR connected to theprevious RAS, and upon failure to receive a fast binding acknowledgementFBAck message, transitioning from a predictive mode to a reactive mode.2. The method of claim 1, wherein the IP network information gatheringcomprises: detecting, by the MAC layer of the MS, an identifier (ID) ofa neighbor RAS through scanning; generating a new RAS identificationprimitive including the ID of the neighbor RAS, and forwarding the newRAS identification primitive to an IP layer; and upon receipt of the newRAS identification primitive, gathering, by the IP layer, IP networkinformation.
 3. The method of claim 1, wherein the IP networkinformation gathering comprises: sending, by the IP layer, a RouterSolicitation for Proxy (RtSoIPr) message to the upper node of theprevious ACR; receiving a Proxy Router Advertisement (PrRtAdv) messagefrom the upper node of previous RAS in response to the RtSoIPr message;and detecting IP network information from the PrRtAdv message.
 4. Themethod of claim 1, wherein the target RAS determining comprises:selecting a target RAS through MAC message exchange between the MS andthe previous RAS; if the target RAS is selected, generating a target RASselect primitive and forwarding the target RAS select primitive to an IPlayer; upon receipt of the target RAS select primitive, sending, by theIP layer of the MS, a Fast Binding Update (FBU) message to the uppernode of the previous ACR; upon receipt of the FBU message, sending, bythe upper node of the previous ACR, a Handover Initiation (HI) messageto the selected upper node of an ACR connected to the target RAS; uponreceipt of a Handover Acknowledge (HAck) message in response to the HImessage, sending, by the upper node of the previous ACR, a Fast BindingAcknowledgement (FBAck) message to the MS and the upper node of the ACRof the target RAS in response to the FBU message; upon receipt of theFBAck message, generating, by the IP layer of the MS, a link switchprimitive and forwarding the link switch primitive to a MAC layer; uponreceipt of the link switch primitive, forwarding, by the MAC layer ofthe MS, a Handover Indication (MOB_HO-IND) message for actual handoverto the previous RAS; and upon receipt of the MOB_HO-IND message, ending,by the previous RAS, service to the MS.
 5. The method of claim 1,wherein the tunneling comprises: completing MAC layer handover byperforming an IEEE 802.16 network entry procedure; after the completionof the MAC layer handover, generating, by the MAC layer of the MS, alink activation (Link_Up) primitive and forwarding the Link_Up primitiveto the IP layer to notify possibility of receiving data through a newlink; and upon receipt of the Link_Up primitive, sending a Fast NeighborAdvertisement (FNA) message to the upper node of an ACR connected to thetarget RAS.
 6. The method of claim 1, wherein the tunneling comprises:completing MAC layer handover by performing an IEEE 802.16 network entryprocedure; after the completion of the MAC layer handover, generating,by the MAC layer of the MS, a Link_Up primitive and forwarding theLink_Up primitive to the IP layer to notify possibility of receivingdata through a new link; upon receipt of the Link_Up primitive, sendingan FNA message to the upper node of an ACR connected to the target RAS;and upon receipt of the FNA message, testing, by the upper node of theACR of the target RAS, uniqueness of a New Care-of-Address (NCoA). 7.The method of claim 6, wherein the FNA message comprises an FBU message.8. The method of claim 6, wherein if the NCoA is unique, the upper nodeof the previous ACR and the upper node of the ACR connected to thetarget RAS generate a tunnel by exchanging the FBU message and the FBAckmessage with each other, and form a packet.
 9. The method of claim 6,wherein if the NCoA is not unique, the upper node of the ACR connectedto the target RAS receives from the upper node of previous ACR a ProxyRouter Advertisement (PrRtAdv) message including a negativeacknowledgement (NACK) message, and discards the FBU message.
 10. Themethod of claim 1, wherein, when the MS enters predictive mode, thetunneling further comprises sending a handover request to the previousRAS and receiving a handover response from the previous RAS.
 11. Themethod of claim 10, wherein the receiving of the handover response fromthe previous RAS comprises receiving the handover response to the linklayer of the MS, and notifying the IP layer of the MS that the handoverresponse has been received.
 12. The method of claim 1, wherein, when theMS enters the reactive mode, the method further comprises sending a FastNeighbor Advertisement (FNA) to a target ACR connected to the target RASto indicate to the target ACR the identity of the previous ACR.